OXIDATION PROCESS

Abstract: A process for oxidizing a sulphite-ion-containing and/or hydrogen-sulphite-ion-containing aqueous solution in the presence of lime, preferably in wet flue-gas desulphurisation. In the process, lime, preferably calcium carbonate, is added to at least a part of the aqueous solution, and this part of the solution is then contacted with an oxygen-containing gas, such as air, before any precipitation of calcium sulphite has occurred. The oxygen-containing gas should be blown into the solution within 1 min of the addition of lime, preferably within 30 s, and most preferred within 5 s. It is further preferred that the solution be vigorously agitated, e.g. by means of a stirrer or a pump, when the process is implemented. The added lime should have a minimal particle size, and the average particle size preferably should not exceed about 30 mu m. It is preferred that the process be implemented at a temperature of about 40-60 DEG C.

Description: OXIDATION PROCESS This invention relates to an oxidation process, more specifically a process for oxidising a sulphite- ion-containing and/or hydrogen-sulphite-ion-containing aqueous solution in the presence of lime, preferably in wet flue-gas desulphurisation. Sulphur dioxide is formed, for instance, when burn¬ ing material containing sulphur or sulphur compounds, such as coal, oil and refuse which contains rubber and kitchen scraps. As used herein, the term "sulphur dioxide" also encompasses any sulphur trioxide that may be present. Gases containing sulphur dioxide, such as flue gases resulting from the burning of coal, have to be cleaned in order not to pollute the environment. This is done with the aid of wet flue-gas desulphurisation, in which the gases are washed in a gas washer or scrubber, where an aqueous washing liquid is divided into droplets and contacted with the gas to be cleaned, such that the sulphur dioxide is absorbed by the washing liquid. When thus absorbed, the sulphur dioxide will be present as sulphurous acid (H2SO3), hydrogen sulphite ions (HS03 ~ ) and/or sulphite ions (SO32-), the relative proportions thereof depending on the pH at the absorption. At a pH below about 2, the absorbed sulphur dioxide is present chiefly as sulphurous acid; at a pH of from about 2 to about 7, the absorbed sulphur dioxide is present chiefly as hydrogen sulphite ions; and at a pH above about 7, the absorbed sulphur dioxide is present chiefly as sul¬ phite ions. The aqueous absorption liquid also contains an absorbent, which in the invention consists of lime. By "lime" is here meant calcium carbonate (CaC03> or slaked lime (Ca(OH)2). Calcium carbonate (limestone) is preferred in actual practice, among other things for reasons of economy. To have optimum effect, the lime should be in the form of a powder of minimal particle size. The admixture of the lime absorbent with the aqueous absorption liquid will result in a slurry con¬ taining calcium ions, which may form sparingly-soluble calcium sulphite with the sulphite ions present. This is undesirable, calcium sulphite being neither a chemically stable product nor a useful end product. Instead, the aimed-at end product is gypsum, i.e. calcium sulphate (CaS04). To obtain this aimed-at end product, any sul- phite ions or hydrogen sulphite ions present in the solution have to be oxidised to sulphate ions. In the prior art, sulphite or hydrogen sulphite is oxidised to sulphate by dispersing air in the absorption liquid. This can be done in many ways. In modern conventional techniques, the absorption liquid as a whole is first oxidised, and fresh calcium carbonate slurry is then added. The purpose of perform¬ ing the oxidation before the addition of calcium carbo¬ nate is to avoid the precipitation of calcium sulphite. The pH of the absorption liquid decreases on the absorp¬ tion of sulphur dioxide, to decrease even further when the sulphite is oxidised to sulphate. The thus-obtained low pH level has the disadvantage of entailing a low rate of the oxidation of sulphite ions to sulphate ions, the oxidation rate decreasing as the pH is reduced. Fur¬ thermore, the low pH value promotes the hydrogen sul¬ phite ions at the expense of the sulphite ions, which also has a negative effect on the rate of oxidation of the sulphite ions. In another known variant of sulphite oxidation, the same volume of liquid is used for the oxidation and the pH adjustment. Thus, absorption liquid is collected at the bottom of the scrubber, and lime (limestone) is added to the liquid while blowing air into it. This variant is disadvantageous in that a considerable volume of liquid is required at the bottom of the scrubber tower in order to attain a satisfactory degree of oxi¬ dation, i.e. conversion of sulphite to sulphate. If the volume of liquid is too small, i.e. the residence time too short, the degree of oxidation becomes unsatisfac- tory, resulting in the precipitation of calcium sulphite and thus in operational disturbances. Moreover, a con¬ siderable volume of liquid is required also for achiev¬ ing satisfactory dissolution of calcium carbonate and satisfactory crystallisation of gypsum. In practice, liquid volumes of about 1000-2000 m^ are commonly used, liquid volumes of up to 6000 m^ being sometimes used. However, such large volumes of liquid make efficient dispersal of the oxidation air difficult, thereby increasing the risk that some of the sulphite escapes oxidation. In another prior-art variant of sulphite oxidation in wet flue-gas desulphurisation, lime (calcium carbo¬ nate) is first added, whereupon the oxidation is per¬ formed. However, sparingly-soluble calcium sulphite is precipitated when the lime is added first. A solid phase, such as precipitated calcium sulphite, is oxidis¬ ed very slowly, since the oxidation is constrained by the concentration of sulphite ions and/or hydrogen sul¬ phite ions in the solution, i.e. by the rate of dissolu- tion of calcium sulphite, and not by the rate of the oxidation reaction. Efforts have previously been made to obviate this inconvenience by dissolving the precipi¬ tated calcium sulphite prior to oxidation by adding sul¬ phuric acid to the solution, thereby to enable subse- quent oxidation by air being blown into the solution. This process not only involves an additional step, but also is more expensive owing to the required addition of sulphuric acid. Moreover, the addition of sulphuric acid augments the precipitation of calcium sulphate. With the aim of obviating, inter alia, the draw¬ backs of the above process involving an addition of sul- phuric acid, DE 33 12 388 discloses a technique in which no sulphuric acid is added, but precipitated calcium sulphite is oxidised to calcium sulphate in a special oxidation column at a pH above 6.5 by heating the absorption solution to 60-90° and blowing air into it. It goes without saying that it is energy-consuming and costly to thus heat the absorption solution to a tem¬ perature of 60-90° According to the present invention, it has now been found that it is possible to remedy the drawbacks of the prior art by first adding lime, and then immediately contacting the resulting solution with an oxygen-con¬ taining gas before any precipitation of calcium sulphite has occurred. Thus, the invention provides a process for oxidis¬ ing a sulphite-ion-containing and/or hydrogen-sulphite- ion-containing aqueous solution in the presence of lime, preferably in wet flue-gas desulphurisation, said pro¬ cess being characterised by adding lime to at least a part of the solution, and contacting this part of the solution with an oxygen-containing gas before calcium sulphite has precipitated. Other distinctive features of the invention appear from the appended claims. Without preference to any specific theory, it is assumed that the inventive oxidation process functions as follows. Sulphur dioxide absorbed in an aqueous solution is present chiefly as hydrogen sulphite ions (HS03 ~ ) at a pH of about 2-7, and chiefly as sulphite ions (Sθ3 2~ ) at a pH above about 7. Sulphite ions and hydrogen sulphite ions are oxidised to sulphate ions by reacting with dissolved oxygen in accordance with the reaction formulae I and II below. 2- 1 2- so + -o → so (I) 3 2 2 4 and 2 - HSO + -O → H + + SO ( II ) 3 2 2 4 The oxidation reactions I and II are catalysed by transition metal ions, e.g. cobalt ions and/or copper ions acting as catalysts in an alkaline solution, while iron ions and manganese ions are the most effective catalysts in an acid solution. The rate of the oxidation reaction I above is affected by the concentration of sulphite ions, but is essentially unaffected by the con¬ centration of oxygen at the low concentrations of sul¬ phite ions prevailing in calcium-based absorption pro¬ cesses. The rate of the reactions I and II increases as the temperature increases, and also increases with the pH value, showing a steep increase when pH exceeds about 5. However, it should be taken into consideration that the solubility of calcium sulphite (CaSθ3) decreases as the pH increases. To increase the rate of oxidation by increasing the pH of the solution is thus only possible up to a certain limit in the case of solutions contain¬ ing calcium ions, since the precipitation of calcium sulphite becomes predominant above a certain pH value. The break point, at which an increase of the pH value no longer results in an increase but in a decrease of the oxidation rate owing to the augmented precipitation of calcium sulphite, occurs at a pH of about 4.5. Since the lime (calcium carbonate) is added, in the invention, to the solution which has absorbed sulphur dioxide and has an acid pH of 3-5, the lime is rapidly dissolved, accom- panied by the escape of carbon dioxide and an increase of the pH value of the solution. The increase of the pH value of the solution results in a shift from hydrogen sulphite ions to sulphite ions in the solution. For a brief period of time, the solution is supersaturated with calcium sulphite, i.e. contains plenty of sulphite ions. Since the pH of the solution in addition has been c" 6 raised to above the critical value 5 by the addition of lime, the rate of oxidation is very high, and essential¬ ly all the sulphite ions can thus be oxidised to sul¬ phate ions. However, the oxidation must take place imme- diately, since there is otherwise a risk that the super- saturation with calcium sulphite is precipitated in the form of calcium sulphite crystals. Therefore, the inven¬ tion is distinguished by the oxidation to sulphate beάng performed after lime has been added to the solution bat before any precipitation of calcium sulphite has occur¬ red. The time elapsing between the addition of lime and the precipitation of calcium sulphite varies depending on temperature, hydrogen sulphite concentration or sulphite concentration, lime concentration, particle size of the lime, agitation of the solution, and so forth. In general, however, calcium sulphite is preci¬ pitated within 1 min, unless the oxidation reaction has been carried out by then. Thus, this means that the solution should be contacted with the oxygen-containing gas within 1 min of the addition of lime. However, this lapse of time is often too long to avoid the precipita¬ tion of calcium sulphite, and it is thus preferred that the solution is contacted with the oxygen-containing gas within 30 s of the addition of lime, more preferred within 10 s, and most preferred within 5 s. Best of all, the solution is brought into contact with the oxygen- containing gas immediately and within 3 s of the addi¬ tion of lime. To facilitate its dissolution, the lime should, as mentioned in the foregoing, be in the form of a powder of minimal particle size, and it is preferred that the average particle size of the lime does not exceed about 100 μm, and most preferred does not exceed about 30 μm. As also mentioned before, the term "lime" as used herein is meant to encompass calcium carbonate and slaked lime. According to the invention, the lime preferably consists of calcium carbonate (limestone). The lime is added to the absorption solution as a powder or, which is preferred, as a concentrated lime slurry. The amount of lime added depends on the amount of sulphur dioxide absorbed by the absorption solution and should be such that it is equimolar with the amount of sulphur dioxide absorbed or is present in a certain excess, e.g. in a molar excess of up to about 50%. Usually, the lime is added in such an amount that the absorption solution contains about 5-20% by weight of solids. The temperature in the inventive process is not critical, and the process may be implemented at ambient temperature or, which is preferred, at a slightly ele¬ vated temperature. It is especially preferred that the process be implemented at a temperature of about 40-60° C. Normally, such a temperature is obtained automati- cally when the absorption solution is contacted with the hot flue gases from which sulphur dioxide is absorbed. The oxygen-containing gas used for performing the oxidation to sulphate may be pure oxygen, but normally consists of air for reasons of practicality and economy. In order to contact the oxygen-containing gas with the absorption solution and perform the oxidation, the oxy¬ gen-containing gas is preferably injected into the solu¬ tion through one or more nozzles. It is then desirable that the oxygen-containing gas be as finely divided as possible in the solution, thereby to optimise the con¬ tact between the oxygen-containing gas and the solution and thus facilitate oxidation. In order to create inti¬ mate contact between the oxygen-containing gas and the solution, to disperse the lime particles and to obstruct the crystallisation of calcium sulphite, it is further highly advantageous to agitate or stir the solution. The injection of the oxygen-containing gas through nozzles may achieve or contribute to such agitation, but it is, according to the invention, preferred that the solution be agitated also in some other manner, e.g. by means of stirrers, pumps, shaking or vibrations. It will be appreciated that, from a purely practi¬ cal point of view, it is difficult when dealing with large volumes of liquid to contact the oxygen-containing gas with the whole solution before calcium sulphite is precipitated. To this end, it is, in accordance with the invention, preferred that merely a part of the solution is treated, such that it is only the volume of liquid that has just absorbed sulphur dioxide in the scrubber that is collected and supplied with lime, followed by oxidation through the injection of air, the solution being vigorously agitated all the while. This is done separately from the subsequent precipitation and sedi¬ mentation of calcium sulphate (gypsum). After oxidation, the solution is drawn off to a container where calcium sulphate (gypsum) is precipitated and sedimented. Thus, the reason for it being, according to the invention, preferred to merely treat a minor amount of the total solution at a time is that it may thus be ensured that precisely that part of the solution to which lime has been added is directly contacted with the oxygen-con¬ taining gas for oxidation purposes before any precipita¬ tion of calcium sulphite has occurred. If the treatment of a larger amount of the total solution can be secured, e.g. by providing a plurality of means for adding lime, injecting the oxygen-containing gas and agitating the solution, there is naturally nothing to prevent that a large amount of the total solution, or even the whole solution, be treated at one time. In order to further elucidate the invention, a few non-restricting embodiments will now be described with reference to the accompanying drawing. In the drawing, Fig. 1 is a schematic view of a currently preferred device for implementing the inven¬ tive process, and Fig. 2 is a partial top view of the device along the line II-II in Fig. 1. The device illustrated in Fig. 1 consists of a scrubber 1 having an inlet 2 for uncleaned flue gas 3 and an outlet 4 for cleaned flue gas 5. At the bottom of the device, there is provided a collecting container 6 for an aqueous absorption solution 7. There are further provided conduits 8 and 9, as well as a pump 10 for sup¬ plying the absorption solution to a plurality of nozzles 11. A collecting baffle 12, which is connected to an upwardly and downwardly open cylinder 13, is arranged inside the device, at the bottom thereof. Inside the cylinder, there are provided nozzles 14 for supplying lime from a conduit 15. A stirrer 16, which is driven by a motor 17, is arranged beneath the nozzles 14. Nozzles 18 for supplying oxidation air from a conduit 19 are provided beneath the stirrer 16. The cylinder 13 is concentrically surrounded by another cylinder 21, which has a bottom 20. The upper edge of the cylinder 21 forms an overflow to the container 6. Precipitated gypsum can be drawn off through conduits 23, 24. As shown in Fig. 2, the inner cylinder 13 and the outer cylinder 21 are interconnected by a number of ver¬ tical walls 26 for producing a good liquid flow in the vertical direction. The device illustrated in Figs 1 and 2 operates as follows. Uncleaned flue gas 3 is introduced into the device and contacted with the absorption solution 7 injected through the nozzles 11. The absorption solution tnen absorbs sulphur dioxide from the flue gas, which thereafter leaves through the outlet 4 in the form of cleaned flue gas 5. The absorption solution containing absorbed sulphur dioxide is collected by the baffle 12 and conducted to the cylinder 13, where lime in the form of fine-grained calcium carbonate is injected through the nozzles 14 while the solution is vigorously agitated by means of the stirrer 16. The solution into which lime has been injected is then immediately conducted past the nozzles 18 injecting air into it. Owing to the provision of the baffle 12 and the assembly 13-18, only a part of the absorption solution, namely the part that has just absorbed sulphur dioxide in the scrubber, is treated by lime addition and oxidation. While still being vigorous- ly agitated, the solution which has absorbed sulphur dioxide and into which lime and air have been injected, ascends in the gap between the cylinder 13 and the cylinder 21, and is then fed over the upper edge of the cylinder 21 into the container 6. As explained in the foregoing, the oxidation of sulphite to sulphate in the absorption solution is very rapid and takes place before any precipitation of cal¬ cium sulphite has occurred, owing to the conditions created by treating the absorption solution with air immediately after the addition of lime and preferably implementing the process while the solution is being vigorously agitated and mixed. When the absorption solu¬ tion enters the container 6, the oxidation is thus essentially completed, and the sulphate ions react with calcium ions to form gypsum, which is precipitated and can be recovered through outlets 23, 24. By the precipi¬ tation of gypsum, the absorption solution 7 is regene¬ rated and freed of absorbed sulphur dioxide and may then again be used for absorbing sulphur dioxide from flue gases by being recycled to the nozzles 11 by means of the pump 10. The device illustrated in Figs 1 and 2 have but a single stirrer and only one or a few nozzles for sup¬ plying lime and air. As a result, the illustrated device only treats a part of the total absorption solution on one and the same occasion, namely the part injected through the nozzles 11. However, it is to be understood that, instead of a single device for supplying lime and air and agitating the solution, provision can be made of two or more such devices in the container 6 for treating the part of the solution injected through the nozzles 11. Also, it will be appreciated that the volume of liquid treated can be extended to include more than the part injected through the nozzles 11, by the provision of one or more devices for supplying lime and air and agitating the solution in the container 6. Thus, a vary¬ ing amount of the total volume of liquid can be treated at one time, as one desires, but it is normally suffi¬ cient to merely treat the volume of liquid injected through the nozzles 11, as illustrated in the Figure. Example 1 A test was performed with absorption of sulphur dioxide from a flue gas in a device of the type describ¬ ed above, by adding calcium carbonate to the absorption liquid and then immediately blowing air into the liquid while vigorously agitating it all the while. The calcium carbonate added had a such a particle size that 90% of the particles passed through a sieve having a mesh width of 44 μm. The air was added in such an amount that the ratio of atomic mole oxygen to mole SO2 was 5:1. The pH value of the absorption slurry was 5.4 in the recirculation tank. By oxidising the absorbed sulphur dioxide to sulphate in accordance with the invention before feeding it to the slurry in the recirculation tank, the resulting end product obtained a degree of oxidation of sulphite to sulphate of 99.8%, i.e. the oxidation was virtually complete. The degree of separation of sulphur dioxide from the flue gas was 85%. Example 2 (comparative) It was proceeded as in Example 1, but no lime was added to the sulphur dioxide absorbed in the form of sulphite, nor did any immediate injection of air under vigorous agitation take place, i.e. the device lacked the components 12-21 in Fig. 1. Instead, use was made of a conventional process and a conventional device, and the absorption solution containing absorbed sulphur dioxide was supplied to the recirculation tank. Finely- divided calcium carbonate was added to the slurry in the recirculation tank, and oxidation was performed by means of air blown into the tank. The pH value in the recir¬ culation tank was 5.4 in the test. Such an amount of air was blown into the solution that the ratio of atomic mole oxygen to mole SO2 exceeded 10:1. The test only yielded an end product showing a degree of oxidation of sulphite to sulphate of 65%, and a degree of separation of 81% with respect to SO2, i.e. the results were much poorer than in the present invention.

Claims: CLAIMS 1. A process for oxidising a sulphite-ion-contain- ing and/or hydrogen-sulphite-ion-containing aqueous solution in the presence of lime, preferably in wet flue-gas desulphurisation, c h a r a c t e r i s e d by adding lime to at least a part of the solution, and contacting this part of the solution with an oxygen- containing gas in order to oxidise sulphite ions and hydrogen sulphite ions to sulphate ions before any pre¬ cipitation of calcium sulphite has occurred. 2. A process as set forth in claim 1, c h a r ¬ a c t e r i s e d by contacting the solution with the oxygen-containing gas within 1 min of the addition of lime. 3. A process as set forth in claim 2, c h a r ¬ a c t e r i s e d by contacting the solution with the oxygen-containing gas within 30 s of the addition of lime. 4. A process as set forth in claim 3, c h a r ¬ a c t e r i s e d by contacting the solution with the oxygen-containing gas within 5 s of the addition of lime. 5. A process as set forth in any one of the preced¬ ing claims, c h a r a c t e r i s e d by adding air as oxygen-containing gas. 6. A process as set forth in any one of the preced¬ ing claims, c h a r a c t e r i s e d by adding, to the solution, lime having an average particle size not exceeding 100 μm, preferably not exceeding about 30 μm. 7. A process as set forth in any one of the preced¬ ing claims, c h a r a c t e r i s e d by adding calcium carbonate as lime. 8. A process as set forth in any one of the preced¬ ing claims, c h a r a c t e r i s e d by agitating the solution while the lime is added and the solution is contacted with the oxygen-containing gas. 9. A process as set forth in claim 8, c h a r ¬ a c t e r i s e d by agitating the solution by means of a stirrer. 10. A process as set forth in claim 8 or 9, c h a r a c t e r i s e d by agitating the solution by blowing the oxygen-containing gas into it.

Inventors: BRINGFORS NILS, BRINGFORS, NILS
Applicants: BRINGFORS NILS, FLAEKT AB, ABB FLAEKT AB, BRINGFORS, NILS
Application year: 1994
Main classification: B01D53
IPC codes: B01D53, C01F11